In the paper manufacturing industry, an increasing number of high-capacity paper machines with speeds of up to 2,000 m/min and operating widths exceeding 10 m are used. As a general rule, the sheet forming unit is configured as a double-cloth former, and in many cases, as a split former. In such machines, the sheet forming process takes place immediately between two paper making cloths in a relatively short drainage zone. Because of this short distance and the high production speed, the time for sheet forming is reduced to a few milliseconds. The solid component or dry content of the fiber suspension must be increased from about 1 percent to approximately 20 percent. The significance for the paper making wire cloths is that they must possess a very high drainage capacity, and yet leave no marks in the paper while providing high fiber support.
Another important point is the transverse stability of the cloth tension, which stability is important for the uniformity of the thickness and water content profile of the paper web. The requirements set in this connection have a very high precision in the case of modern machines with large operating widths. Forming strips, which are mounted alternately on the backing sides of the cloths and are pressed against them, are now used in the sheet forming zone with increasing frequency to improve formation. This arrangement results in rapidly changing bowing of the covering of the wire cloths in the longitudinal direction.
At present, an effort is usually made to meet these requirements by using composite fabrics. One composite fabric used for the purpose is described, for example, in DE 42 29 828 C2. The conventional paper making machine wire cloth used for the purpose has two stacked cloth fabrics which make up at least one layer and are interconnected by binding threads extending in the transverse and/or longitudinal directions. One of the cloth fabrics is configured as a definition fabric having the mechanical properties of the composite fabric with respect to extension and rigidity. The other cloth fabric is configured as a reaction fabric characterized by greater extension and lower rigidity than the definition fabric. The cloth fabrics are formed of warp and weft threads, these threads being interconnected by additional binding threads. Internal wear, especially wear of the binding threads, is counteracted to increase the service life of the composite fabric. Undesirable separation of the cloth fabric layers is prevented over a longer period as a result of design of the cloth fabric layers as reaction and definition cloths. The internal wear of a composite fabric is due particularly to the circumstance that the individual cloth fabric layers are stretched or buckled to a varying extent during reversals of the wire cloth, as occur in the area of guide rollers or wire section over which the composite cloth is guided.
Since the binding threads do not belong to the fabric structure, but rather are independent components, they are kept thin in diameter to disrupt the drainage as little as possible. When suitable high stresses occur, the possibility exists that the thin binding threads will split and the connection between the cloth fabrics will be lost. In the case of a generic paper making machine wire cloth as specified in EP 0 432 413 B1, which also is configured as a composite fabric, two threads of the fabric itself can be used and can be woven into the other fabric layer to form X-shaped intersections in order to avoid the disadvantages of the state of the art as described. However, undesirable stiffening of the fabric results from the accumulation itself of the intersections in the transverse direction. Significant differences in length may occur especially over longer weaving lengths. These differences may in turn be expressed in differences in tension, with the result that the binding threads specific to the cloth may also break and may result in failure of the conventional paper making machine wire cloth. In addition, with this conventional special weave, it is more or less possible only to process transverse threads of one type, that is, transverse threads with more or less the same diameter compositions. This limitation reduces the possibility of effective support on the backing side. Production of the conventional composite fabrics as described is also costly.
EP 0 698 682 A1 discloses a fabric formed of a system of face wefts, back wefts, and warps, the latter being made up of a system pair of first and second types of warp threads. The warp threads of the first type are interwoven with the face wefts and, at predetermined distances, intermittently with the back wefts. Warp threads of the second type extend between the face and back wefts, and bind with the face wefts at the point at which the first warp thread belonging to the pair binds with the back weft. Consequently, the warp threads are then positioned one directly above the other, except at the point at which the second warp threads bind with the face weft. As a result of this proximate state of the art, in the case of a paper making machine wire cloth, while surface uniformity on the paper side is improved, separation of the layers or shifting of the layers of fabric in relation to each other cannot be completely excluded.